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SymmetryThe World of Organic Molecules
What is Symmetry?
Symmetry is likeness of shape or arrangement about a center.
Does this Design Have Symmetry?
Symmetry About a PlaneSymmetry About a Plane
Anything that contains an internal mirror Anything that contains an internal mirror plane is symmetric.plane is symmetric.
Anything that lacks an internal mirror plane Anything that lacks an internal mirror plane is asymmetric.is asymmetric.
ChiralityChirality
Asymmetric objects have an interesting Asymmetric objects have an interesting property called Chirality which means property called Chirality which means “handedness”.“handedness”.
Chiral objects can have two different forms, Chiral objects can have two different forms, a right-handed and left-handed form which a right-handed and left-handed form which are mirror images of each other but are are mirror images of each other but are not exactly alike.not exactly alike.
Examples of Chiral ObjectsExamples of Chiral Objects
Your Hands and FeetYour Hands and Feet
Gloves and ShoesGloves and Shoes
Golf ClubsGolf Clubs
Computer keyboardComputer keyboard
Are the Following Objects Are the Following Objects Chiral or Achiral?Chiral or Achiral?
Baseball BatBaseball Bat
RefrigeratorRefrigerator
CarCar
BicycleBicycle
Your FaceYour Face
ForkFork
NailNail
ScrewScrew
Chiral Molecules?Chiral Molecules?
A carbon atom with four different groups A carbon atom with four different groups bonded to it creates a stereocenter in the bonded to it creates a stereocenter in the molecule.molecule.
Most molecules which contain one or more Most molecules which contain one or more stereocenters lack an internal plane of stereocenters lack an internal plane of symmetry and are therefore chiral.symmetry and are therefore chiral.
EnantiomersEnantiomers
The right-handed and left-handed forms of a The right-handed and left-handed forms of a chiral molecule make up a pair of stereoisomers chiral molecule make up a pair of stereoisomers called enantiomers.called enantiomers.
Enantiomers are not identical, but they have the Enantiomers are not identical, but they have the same physical properties such as melting point same physical properties such as melting point boiling point, etc.boiling point, etc.
So why do we care about this subtle form of So why do we care about this subtle form of stereoisomerism?stereoisomerism?
Chirality in the Biological WorldChirality in the Biological World
When chiral organic compounds interact with When chiral organic compounds interact with living systems, the two enantiomers often living systems, the two enantiomers often interact very differently.interact very differently.
This is because living systems are chiral at This is because living systems are chiral at many different levels in many ways:many different levels in many ways:- our hands and feet are chiral (of course)- our hands and feet are chiral (of course)- our taste buds are chiral- our taste buds are chiral- our olfactory sensors are chiral- our olfactory sensors are chiral
- our enzymes are chiral- our enzymes are chiral
Carvone
O
CH3
CH3
H
CH3
H
O
H3C
Carvone
O
CH3
CH3
H
CH3
H
O
H3C
Left-handed Carvone
Smells like caraway
Right-handed Carvone
Smells like spearmint
Limonene
CH3
CH3
H
CH3
H
H3C
Limonene
CH3
CH3
H
CH3
H
H3C
Left-handed Limonene
Smells HarshTurpentine-like
Right-handed Limonene
Smells like fresh organges
Aspartame
H2N
N
O
O
O
H
H
O
OH
H
H2N
N
O
O
O
H
H H
O
OH
Aspartame
H2N
N
O
OH
O
H
H
O
OH
H
Left-handed Aspartame"Nutrasweet"160 times sweeter than sugar
H2N
N
O
OH
O
H
H H
O
OH
Right-handed AspartameNot at all sweetslightly bitter
Ibuprofen
H3C
O
OH
CH3
O
HOH H
Ibuprofen
H3C
O
OH
CH3
O
HO
Left-handed IbuprofenPowerful Pain Killer and Anti-inflamatory Drug
H H
Right-handed IbuprofenNo Drug Activity
Summary
• Molecules which lack symmetry are Chiral
• Chiral molecules have right-handed and left-handed forms
• Right-handed and left-handed molecules interact with living systems in very different ways.– Different smell– Different taste– Different drug effects
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